Suspensions of particulate materials in gaseous and liquid media are an important natural and man-made phenomenon. Smokes and paints are some familiar examples of these mixtures. Other examples include air samples to test for the presence and concentrations of asbestos, silica, cotton fibers, and coal dust. To further define these suspensions, it is usually necessary to analyze them qualitatively and quantitatively. One of the common methods for analysis of the gaseous or liquid media suspensions begins by separating the solids using a filtration operation. This process simplifies the analysis by collapsing the two phase, three-dimensional system into a one phase, two-dimensional system. The gas or liquid will be separated from the solid particulate, and a concentration of the particulate is achieved. The settling out or impingement of particulate results in a similar potential sample which has been selectively separated, when those methods are used. Materials retained on the filter or other deposition surface are then available for analysis by a variety of techniques that could not be employed while the particulate was suspended.
The retention of the captured material on the surface must be maintained during the sample's transport and other preparatory steps if meaningful analytical results are to be obtained from the filtration or impingement operations. While the first step in particulate collection has been carefully studied to reveal that the interception, impaction, electrostatic attraction, and van Der Waal force are primary deposition mechanisms, the forces for particle retention are not so well defined as to assure continued interaction under every condition the sample may encounter thereafter. Environmental changes can occur immediately after the sampling operation is concluded. The post-sampling conditions may potentially be as important as the conditions during filtration in producing a representative sample. For example, when the pressure differential on the filter is released, the fluid flow ceases and the particulate might become less tightly held to the collecting substrate. The static electric charge established during sampling may change. Evaporation may occur or hygroscopic actions may change surface tensions. For these reasons and others, the particle/filter or particle/surface interaction dynamics may be significantly altered thus affecting the distribution and/or retention of the particulate on the filter.
The mechanisms which maintain the particle in contact with the filter after the initial capture are electrostatic and molecular in nature unless the particulate is entrapped physically in the filtering media. These forces are relatively weak and are very apt to change if the sample is transported. In the transportation process, the sample will likely undergo a certain amount of mechanical shock, vibration, temperature change, humidity change, pressure change and electrostatic force transitions. Gravity may act as a positive or negative fact or in maintaining the particle contact. Carrying the sample upright at all times to its destination will reinforce the association but will not guarantee the sample integrity. Carrying the sample on the filter in any other orientation will be a negative factor in this particle/filter association. These same potentially disruptive effects are present on both filter and surface collected samples whatever the mode of transportation and regardless of the trip length. The only variable is the degree of disruption in a given instance. These stresses will dislodge the particulate if they individually or collectively exceed the particle attachment forces.
With the exception of some samples destined for X-ray analysis or impactor samples collected on greased plates, little attention is paid to insuring that the samples are fixed in place before committing them to the mail or the other transport system. Presently, tens of thousands of samples are at risk each year with only relatively passive attempts being made to protect them by attaching warning labels to the package such as "handle with care" or "this side up". When these requests are made, they may not be recognized in time or acknowledged fully and thus are not totally satisfactory solutions to this problem. Furthermore, a mere warning may not be sufficient protection to the materials while in shipment. Since the particulates in a sample are often microscopic in size, it would take a special microscopic survey of the filter and containment surfaces to possibly detect a displacement of a sample. Even then, proving a redistribution on the filter surface would be difficult. Obtaining a representative sample of the material is extremely important to the data interpretation and deductive conclusion part of any study, since: (a) only small portions of the filter may be analyzed and (b) large multiplication factors are applied to the portions analyzed to calculate the constitution of the entire sample. Any loss or addition of material to the section analyzed would be an error which would multiply proportionally.
Presently, particulate samples are coated with evaporated carbon or metal as the first step in the laboratory to fix them in place for the electron microscopy analytical process. Optical microscope samples are fixed to a slide/coverslip arrangement. However, fixing the sample at the laboratory may be too late for preserving the original particle distribution.